Attentional Capture by Motion Onsets Is Modulated by Perceptual Load

Article excerpt

The onset of motion captures attention during visual search even if the motion is not task relevant, which suggests that motion onsets capture attention in a stimulus-driven manner. However, we have recently shown that stimulus-driven attentional capture by abruptly appearing objects is attenuated under conditions of high perceptual load. In the present study, we examined the influence of perceptual load on attentional capture by another type of dynamic stimulus: the onset of motion. Participants searched for a target letter through briefly presented low- and high-load displays. On each trial, two irrelevant flankers also appeared, one with a motion onset and one that was static. Flankers defined by a motion onset captured attention in the low-load but not in the high-load displays. This modulation of capture in high-load displays was not the result of overall lengthening of reaction times (RTs) in this condition, since search for a single low-contrast target lengthened RTs but did not influence capture. These results, together with those of previous studies, suggest that perceptual load can modulate attentional capture by dynamic stimuli.

In any visual scene, there is more information present than the visual system can process at one time. As a result, specialized attentional mechanisms have evolved that allow us to select and process only the most relevant information in a scene. For example, attention can be selectively focused on the computer screen during typing, while extraneous information in the surrounding environment is ignored. This voluntary-or goal-directed-form of attentional control is central to our ability to carry out goals effectively in complex visual environments. At the same time, it is also possible for salient aspects of stimuli outside of our current focus to capture our attention, redirecting the focus of attention in a stimulus-driven manner. Although stimulus factors play a role in both goaldirected and stimulus-driven attentional control, some have argued that stimulus-driven control is automatic and occurs independent of the goals or beliefs of an observer (e.g., Theeuwes, 1994), whereas others have shown that attentional capture is under the top-down control of the observer (Folk, Remington, & Johnston, 1992). Thus, it is not clear under what conditions, if any, capture can be purely stimulus driven.

In a number of recent studies, the onset of motion has been demonstrated to capture attention in a stimulusdriven manner, regardless of the observer's goals (Abrams & Christ, 2003, 2005, 2006; Franconeri & Simons, 2003, 2005). For example, Franconeri and Simons (2003) showed that a variety of dynamic stimuli could capture attention in a stimulus-driven manner, using an irrelevantfeature search task (Jonides & Yantis, 1988). Observers initially viewed a five-item placeholder array, and directly following this, portions of each placeholder were removed to reveal a five-item search array. Prior to the presentation of the search array, one of the items in the placeholder array moved, generating a motion onset. Importantly, this motion onset was equally likely to occur at the location of the target or at one of the distractor locations. Across a number of different types of motion onset (e.g., jitter and looming), search slopes to find a target were shallower when the motion onset occurred at the target location than when it occurred at one of the distractor locations. Because this effect was observed despite the task irrelevance of the motion onset, it was concluded that motion onset captures attention in a purely stimulus-driven manner (Franconeri & Simons, 2003).

In contrast to these results, we have recently demonstrated that salient, task-irrelevant stimuli can be ignored in search arrays that are high in perceptual load (Cosman & Vecera, 2009). Using displays in which we manipulated perceptual load (cf. Lavie, 1995; Lavie & Cox, 1997), we showed that abruptly appearing, task-irrelevant flanker letters affected performance in low-load but not in highload displays. …